Thermostatic switches (thermal switches) are engineered for use in high reliability applications such as Space Science Satellites, Defense Satellites, Commercial Satellites, Manned Space Flight Programs and High-Value Terrestrial Applications. Materials constituting thermal switches (referred to hereafter as “switches”) are developed and fabricated to have long life (20+ years) and high reliability while operating under extreme conditions even where service of the switch is impracticable such as an application within Space and Launch Vehicles.
The switches are bimetallic snap action type. A bimetallic disk actuates by detecting temperature change above or below its operational set points. The disk is made of two dissimilar metals: a low expansion side and a high expansion side. These metals are repeatedly rolled together and annealed to create a high state of reduction. The materials are then punched into disks from strip, formed, heat treated, and tested to meet specific temperature requirements. The result is a precision temperature switch.
The bimetallic disk does not have electrical contacts mounted on it. An armature spring is parallel to the bimetallic disk and urges a set of electrical contacts together to form closed a switch. A mechanical link between the bimetallic disk and the armature spring conveys the force created by triggering the disk to the armature spring thereby opening the contacts. That mechanical link is called a striker pin. Conventionally, the striker pin is mounted on the armature spring and bears against the triggered disk. Triggering the bimetallic disk causes it to snap from a concave to a convex shape striking the striker pin. The pin presses, in turn, the armature to the open contact position.
Alumina (Al2O3) is a preferred material for the striker pin. Its high free energy of formation makes alumina chemically stable and refractory, and hence it finds uses in containment of aggressive and high temperature environments. The high hardness of alumina imparts wear and abrasion resistance. The high volume resistivity and dielectric strength make alumina an excellent electrical insulator. These qualities make it a suitable material for the high temperature and numerous cycles. Unfortunately, alumina is an abrasive material. While fastening prevents the striker pin from wearing into the armature spring, the end of the striker pin bearing against the bimetallic disk often wears or cuts into the surface of the disk over repeated duty cycles. Cycling of the switch and the attendant cutting action of the ceramic on the disk at the disk-to-pin interface affect critical dimensions and generate metallic fragments that might interfere with the operation of the switch.
To stem the wear on the bimetallic disk, a metallic coating is deposited at the point where the striker pin bears against the bimetallic disk. The purpose of the coating is to substitute the smooth lubricious surface of a metal such as nickel for the abrasive surface of the alumina ceramic. The current metal caps are very difficult to place accurately. Unfortunately the placement of the caps is not easily reproducible causing variance in the critical length dimension of the resulting pin. Slightly skewed caps vary the overall length. Epoxy resinous adhesives tend to outgas and degrade in the extreme harsh heated environments the thermal switch is design to operate in.
There is an unmet need in the art for a striker pin with an affixed bearing surface to prevent disk-to-pin wear while maintaining the useful properties of alumina.